Throop Company Meets Greenbook Standards For Volumetric Concrete Mixers
The Greenbook is a set of specifications for public works projects that are not in the California State Highway System or National Highway System specifications.
The Greenbook, officially titled “Standard Specifications for Public Works Construction,” is a reference manual used in public works construction projects in California. It provides standardized specifications and guidelines for various aspects of construction, promoting uniformity and clarity in bidding and construction processes. BTW, the Greenbook is available for purchase in physical and digital formats.
The key aspects of the Greenbook is to standardize specifications across different municipalities and agencies involved in public works projects. It covers a wide range of topics, including general provisions, materials, construction methods, existing improvements, pipeline rehabilitation, traffic control, and more.
Until 2020 volumetric concrete mixers were not part of the Greenbook. The cement associations of California and Nevada took the initiative to get it added. Jeff Throop was part of the team to do the final edits/consultations for the official entry of the volumetric concrete mixer specifications addition into the Greenbook.
To be “approved” to use the Greenbook (Standard Specifications for Public Works Construction), you need to ensure that the project you are working on is located off the State Highway System (SHS) and the National Highway System (NHS). For projects on or in those systems, Caltrans Construction Standards must be used which are specified in both the Greenbook and by Caltrans.
As a producer of concrete using volumentric mixers, now that the Greenbook has the standard included, the Throop Company can produce concrete in any city that uses the Greenbook as their Standard Specification. It makes the opportunity to produce in non-Caltrans projects much easier.
It’s important to note that the Greenbook does not test or approve products; it provides specifications that have general applicability to public works projects.
More Information about the Greenbook
The Standard Specifications for Public Works Construction (popularly known as the “Greenbook”) was first published in 1967. It eliminates conflicts and confusion among the plans and specifications used by local public agencies.
It is the work of an active volunteer committee of professional agency members who are thoroughly versed in the changing technology and advanced thinking of the construction industry.
The new 2024 Edition brings the specifications in the “Greenbook” completely up to date in accordance with best practices and the latest technologies. Arranged in major sections, the “Greenbook” covers General Provisions for public works construction as well as detailed specifications covering:
- Construction Materials including rock materials, concrete, mortar, pipe, water and sewer valves, engineering geosynthetics, and bedding and backfill materials.
- Construction Methods including earthwork, subgrade preparation, treated materials and placement of base materials, roadway surfacing, metal fabrication and construction, pile driving and timber construction, open trench conduit construction, jacking and tunneling, and microtunneling
- Existing Improvements including protection and restoration, manhole adjustment and reconstruction, cold milling, and micro-milling
- Pipeline Systems Rehabilitation including service lateral connection sealing
- Temporary Traffic Control including access and temporary traffic control for construction and maintenance work zones
- Street Light and Traffic Signal Systems including materials and construction
- Landscaping and Irrigation including materials and installation.
The “Greenbook” is published every three years under the oversight of Public Works Standards, Inc. (PWSI). The members of PWSI include the American Public Works Association (APWA), Associated General Contractors of California (AGCC), Engineering Contractors Association (ECA), and Southern California Contractors Association (SCCA).
Greenbook Detailed Specifications for Volumentric Concrete Mixing
Volumetric mixing is a precise and efficient method of producing concrete on-site, allowing for real-time adjustments to the mix composition. This section outlines the standards and requirements for volumetric mixing, proportioning, mixer specifications, calibration, and concrete mixing procedures to ensure consistency, compliance, and high-quality results.
201-1.5.5 Volumetric Mixing.
201-1.5.5.1 General.
Concrete may be proportioned and mixed using a volumetric mixer.
201-1.5.5.2 Proportioning.
- Volumetric mixers shall be capable of proportioning cement, water, aggregate, and additives by volume.
- Aggregates shall be proportioned using a belt feeder that is operated with an adjustable cutoff gate delineated to the nearest quarter increment. The gate opening height shall be readily determinable.
- Cement shall be proportioned by any method that complies with the accuracy tolerance specifications in section 201-1.1.
- Water and liquid admixtures shall be proportioned with a meter or meters.
201-1.5.5.3 Mixer Requirements.
- Volumetrically-proportioned concrete shall be mixed in a mechanically operated mixer. An auger-type mixer may be used.
- The mixer shall be operated uniformly at the mixing speed recommended by the manufacturer.
- Do not use a mixer that has an accumulation of hard concrete or mortar.
- Volumetric mixers shall conform to the following:
- Aggregate feeders shall connect directly to the drive on the cement vane feeder.
- The cement feed rate shall be tied directly to the feed rate for the aggregate and other materials. The ratio of cement to aggregate shall be changed only by changing the gate opening for the aggregate feed.
- The drive shaft of the aggregate feeder shall have a revolution counter reading to the nearest full or partial revolution of the aggregate delivery belt.
- Equipment with components made of aluminium or magnesium alloys that could have contact with plastic concrete during mixing or transporting of the concrete shall not be used.
- The rotating and reciprocating equipment on volumetric mixers with metal guards shall be covered.
- The identifying numbers of volumetric mixers shall be at least 3 inches (75 mm) in height and be located on the front and rear of the vehicle.
- Each mixer shall have metal plates that state the designed usage, the manufacturer’s guaranteed mixed concrete volumetric capacity, and the rotation speed.
- The system shall be equipped with a device that automatically shuts down power to the cement feeder and aggregate belt feeder if the cement storage level is less than 20 percent of the total volume.
- Each aggregate bin shall be equipped with a device that automatically shuts down the power to the cement feeder and the aggregate belt feeder if the aggregate discharge rate is less than 95 percent of the scheduled discharge rate.
- The proportioning device indicators shall be in working order before starting proportioning or mixing and must be visible when standing near the volumetric mixers.
201-1.5.5.4 Mixer Calibration.
- The cutoff gate for each volumetric mixer used and for each aggregate source shall be calibrated.
- Each volumetric mixer shall be calibrated at 3 different aggregate gate settings that correspond to production needs. At least 2 calibration runs for each aggregate gate shall be performed.
- Individual aggregate delivery rate check-runs shall not vary by more than 1.0 percent from the mathematical average of all runs for the same gate and aggregate type. Each test run shall be at least 1,000 pounds.
- Individual cement delivery rate check-runs shall not vary by more than 1.0 percent from the mathematical average of 3 runs of at least 1,000 pounds each.
- When the water meter operates from 50 to 100 percent of production capacity, the indicated weight of water delivered shall not differ from the actual weight delivered by more than 1.5 percent for each of 2 runs of 300 gallons for pavement or 75 gallons for structures.
- Calibrate the water meter in accordance with California Test 109. The water meter shall be equipped with a resettable totalizer and must display the operating rate.
- Calibration tests for aggregate, cement, and water proportioning devices shall be conducted using a platform scale located at the calibration site.
- Platform scales for weighing test-run calibration material shall have a maximum capacity of 2.75 tons with maximum graduations of 1 pound. Platform scales shall be error tested within 8 hours of calibrating the volumetric mixer proportioning devices.
- A witness scale that is within 2 graduations of the test weight load shall be furnished.
- The witness scale shall be available for use at the production site throughout the production period.
- Equipment needed for the calibration of proportioning systems shall remain available at the production site throughout the production period.
- Volumetric mixers shall be equipped such that accuracy checks can be made.
- Proportioning devices shall be recalibrated at a minimum of every 90 days or whenever the source or type of any ingredient is changed.
- Each time 55 tons of cement passes through the volumetric mixer, a 2-run spot calibration of only the cement proportioning system shall be performed. If the spot calibration shows that the cement proportioning system does not comply with the specifications, a full calibration of the cement proportioning system shall be completed before resuming production.
201-1.5.5.5 Mixing Concrete.
At the time of batching volumetrically-proportioned concrete:
- Aggregates shall be dried and drained to a stable moisture content. Aggregates with visible separation of water from the aggregate shall not be proportioned.
- Free moisture content of the fine aggregate shall not exceed 8 percent of its saturated surface-dry weight.
- If the proportioning plant has separate supplies of the same size group of aggregate with different moisture contents, specific gravity, or surface characteristics affecting workability, one supply shall be exhausted before using another supply.
- Ice shall not be directly used to cool volumetrically-proportioned concrete. If ice is used to cool the water used in the mix, it shall be melted before entering the mixer.
- The cement shall be proportioned and charged into the volumetric mixer such that there is no variance of the required quantity due to conditions such as wind or accumulation on equipment.
- The volume of material in a volumetric mixer shall not exceed that which allows complete mixing. The volume of material shall be reduced if complete mixing is not achieved.
- Mixing shall continue until a homogeneous mixture is produced during discharge.
Volumetric mixing provides enhanced precision, flexibility, and efficiency in concrete production. By adhering to strict proportioning, mixer requirements, calibration procedures, and mixing guidelines, this method ensures high-quality, consistent results. Proper calibration and compliance with these standards are essential for maintaining durability, performance, and safety in volumetric concrete applications.
